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Fair Value Measurements - Quantitative Disclosures of Fair Value Assets (Details) - USD ($)
$ in Thousands
Mar. 31, 2024
Dec. 31, 2023
AFS fixed income securities $ 7,583,504 $ 7,499,197
Equity securities 194,320 187,155
Short-term investments 247,883 309,317
Fair Value, Recurring    
AFS fixed income securities 7,583,504 7,499,197
Equity securities 194,320 187,155
Short-term investments 247,883 309,317
Total assets measured at fair value 8,025,707 7,995,669
U.S. government and government agencies    
AFS fixed income securities 141,826 205,035
U.S. government and government agencies | Fair Value, Recurring    
AFS fixed income securities 141,826 205,035
Foreign government    
AFS fixed income securities 9,229 9,803
Foreign government | Fair Value, Recurring    
AFS fixed income securities 9,229 9,803
Obligations of states and political subdivisions    
AFS fixed income securities 538,986 585,965
Obligations of states and political subdivisions | Fair Value, Recurring    
AFS fixed income securities 538,986 585,965
Corporate securities    
AFS fixed income securities 2,795,050 2,711,239
Corporate securities | Fair Value, Recurring    
AFS fixed income securities 2,795,050 2,711,239
CLO and other ABS    
AFS fixed income securities 1,897,085 1,834,827
CLO and other ABS | Fair Value, Recurring    
AFS fixed income securities 1,897,085 1,834,827
RMBS    
AFS fixed income securities 1,511,959 1,477,483
RMBS | Fair Value, Recurring    
AFS fixed income securities 1,511,959 1,477,483
CMBS    
AFS fixed income securities 689,369 674,845
CMBS | Fair Value, Recurring    
AFS fixed income securities 689,369 674,845
Common stock: | Fair Value, Recurring    
Equity securities 192,510 185,339
Preferred stock | Fair Value, Recurring    
Equity securities 1,810 1,816
Quoted Prices in 
Active Markets for
Identical Assets/
Liabilities (Level 1) | Fair Value, Recurring    
AFS fixed income securities 33,530 34,056
Equity securities 38,143 22,398
Short-term investments 246,904 308,512
Total assets measured at fair value 318,577 364,966
Quoted Prices in 
Active Markets for
Identical Assets/
Liabilities (Level 1) | U.S. government and government agencies | Fair Value, Recurring    
AFS fixed income securities 33,530 34,056
Quoted Prices in 
Active Markets for
Identical Assets/
Liabilities (Level 1) | Foreign government | Fair Value, Recurring    
AFS fixed income securities 0 0
Quoted Prices in 
Active Markets for
Identical Assets/
Liabilities (Level 1) | Obligations of states and political subdivisions | Fair Value, Recurring    
AFS fixed income securities 0 0
Quoted Prices in 
Active Markets for
Identical Assets/
Liabilities (Level 1) | Corporate securities | Fair Value, Recurring    
AFS fixed income securities 0 0
Quoted Prices in 
Active Markets for
Identical Assets/
Liabilities (Level 1) | CLO and other ABS | Fair Value, Recurring    
AFS fixed income securities 0 0
Quoted Prices in 
Active Markets for
Identical Assets/
Liabilities (Level 1) | RMBS | Fair Value, Recurring    
AFS fixed income securities 0 0
Quoted Prices in 
Active Markets for
Identical Assets/
Liabilities (Level 1) | CMBS | Fair Value, Recurring    
AFS fixed income securities 0 0
Quoted Prices in 
Active Markets for
Identical Assets/
Liabilities (Level 1) | Common stock: | Fair Value, Recurring    
Equity securities 36,333 20,582
Quoted Prices in 
Active Markets for
Identical Assets/
Liabilities (Level 1) | Preferred stock | Fair Value, Recurring    
Equity securities 1,810 1,816
Significant Other Observable Inputs (Level 2) | Fair Value, Recurring    
AFS fixed income securities 6,994,279 6,914,306
Equity securities 0 0
Short-term investments 979 805
Total assets measured at fair value 6,995,258 6,915,111
Significant Other Observable Inputs (Level 2) | U.S. government and government agencies | Fair Value, Recurring    
AFS fixed income securities 108,296 170,979
Significant Other Observable Inputs (Level 2) | Foreign government | Fair Value, Recurring    
AFS fixed income securities 9,229 9,803
Significant Other Observable Inputs (Level 2) | Obligations of states and political subdivisions | Fair Value, Recurring    
AFS fixed income securities 531,239 578,131
Significant Other Observable Inputs (Level 2) | Corporate securities | Fair Value, Recurring    
AFS fixed income securities 2,507,554 2,413,907
Significant Other Observable Inputs (Level 2) | CLO and other ABS | Fair Value, Recurring    
AFS fixed income securities 1,636,988 1,589,514
Significant Other Observable Inputs (Level 2) | RMBS | Fair Value, Recurring    
AFS fixed income securities 1,511,959 1,477,483
Significant Other Observable Inputs (Level 2) | CMBS | Fair Value, Recurring    
AFS fixed income securities 689,014 674,489
Significant Other Observable Inputs (Level 2) | Common stock: | Fair Value, Recurring    
Equity securities 0 0
Significant Other Observable Inputs (Level 2) | Preferred stock | Fair Value, Recurring    
Equity securities 0 0
Significant Unobservable Inputs (Level 3) | Fair Value, Recurring    
AFS fixed income securities 555,695 550,835
Equity securities 1,067 854
Short-term investments 0 0
Total assets measured at fair value 556,762 551,689
Significant Unobservable Inputs (Level 3) | U.S. government and government agencies | Fair Value, Recurring    
AFS fixed income securities 0 0
Significant Unobservable Inputs (Level 3) | Foreign government | Fair Value, Recurring    
AFS fixed income securities 0 0
Significant Unobservable Inputs (Level 3) | Obligations of states and political subdivisions | Fair Value, Recurring    
AFS fixed income securities 7,747 7,834
Significant Unobservable Inputs (Level 3) | Corporate securities | Fair Value, Recurring    
AFS fixed income securities 287,496 297,332
Significant Unobservable Inputs (Level 3) | CLO and other ABS | Fair Value, Recurring    
AFS fixed income securities 260,097 245,313
Significant Unobservable Inputs (Level 3) | RMBS | Fair Value, Recurring    
AFS fixed income securities 0 0
Significant Unobservable Inputs (Level 3) | CMBS | Fair Value, Recurring    
AFS fixed income securities 355 356
Significant Unobservable Inputs (Level 3) | Common stock: | Fair Value, Recurring    
Equity securities 1,067 854
Significant Unobservable Inputs (Level 3) | Preferred stock | Fair Value, Recurring    
Equity securities 0 0
Fair Value Measured at Net Asset Value Per Share    
Equity securities $ 155,100 $ 163,900